The increased complexity and density of semiconductor integrated circuits has motivated great advances in the fabrication techniques used to produce them. One of the several important processes involved in fabricating integrated circuits involves the deposition of electrically conducting metal layers. Most typically, the metal deposition is performed by physical vapor deposition (PVD), also known by the more conventional term of sputtering. At the present time, the most common interconnect metal is aluminum, perhaps alloyed with up to a few percent with copper, magnesium, or silicon
A conventional PVD reactor 10 is illustrated schematically in cross section in FIG. 1, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14 composed of the material to be sputter deposited on a wafer 16 held on a heater pedestal 18. A shield 20 held within the chamber protects the chamber wall 12 from the sputtered material and provides the anode grounding plane. A selectable DC power supply 22 biases the target 0 negatively to about -600 VDC with respect to the shield 20. Conventionally, the pedestal 18 and hence the wafer 16 is left electrically floating.
A gas source 24 of sputtering working gas, typically chemically inactive argon, supplies the working gas to the chamber through a mass flow controller 26. A vacuum system 28 maintains the chamber at a low pressure. Although the base pressure can be held to about 10.sup.7 Torr or even lower, the pressure of the working gas is kept between about 1 and 1000 mTorr.
A computer-based controller 30 controls the reactor including the DC power supply 22 and the mass flow controller 26.
When the argon is admitted into the chamber, the DC voltage between the target 14 and the shield 20 ignites the argon into a plasma, and the resultant positively charged argon ions are attracted to the negatively charged target 14. The ions strike the target 14 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 14. Some of the target particles strike the wafer 16 and are thereby deposited on it, thereby forming a film of the target material.
To provide efficient sputtering, a magnetron 32 is positioned in back of the target 14. It has opposed magnets 34, 36 creating a magnetic field within the chamber in the neighborhood of the magnets 34, 36. The magnetic field traps electrons, and to maintain charge neutrality the ion density also increases to form a high-density plasma region 38 within the chamber adjacent to the magnetron 32.
Another area of recent development in PVD reactors involves the use of a high-density plasma (HDP), often achieved by RF inductive coils placed around the cylindrical walls 12 of the chamber. The inductively coupled RF power creates an intense plasma. One advantage of HDP sputtering is the increased density of argon ions increases the sputtering rate. Another and perhaps more important advantage is that the intense plasma causes a substantial fraction of the sputtered particles to be ionized. Such ionized sputtered particles may be controllably accelerated across the plasma sheath adjacent to the wafer. The resultant forward peak in the velocity distribution, unlike the more normal nearly isotropic distribution for neutral sputtered particles, is effective in causing the sputtering to fill contact and via holes that are narrow and deep, that is, have a high aspect ratio.
For a number of reasons, mostly related to the decreasing dimensions of metal interconnects in advanced integrated circuits, many now believe that aluminum interconnects should be replaced by interconnects of other materials, one of the most prominent being copper. Copper can be easily sputtered in the PVD reactor of FIG. 1 by using a target 14 composed of copper.
It has been recognized that many of the advantages of HDP-PVD can be attained in copper sputtering with the more conventional PVD reactor of FIG. 1 using sustained self-sputtering, as has been explained for example by Fu et al. in U.S. patent application, Ser. No. 08/854,008, filed May 7, 1997, incorporated herein by reference. A reactor 50 illustrated in the cross-sectional view of FIG. 2 includes the reactor described by Fu et al. as well as features of the invention.
The reactor 50 of Fu et al. differs from the conventional PVD reactor 10 of FIG. 1 in a number of features, even aside from the target 14 being composed of copper so as to accomplish copper sputtering. A high-field magnetron 52 is composed of a button magnet 54 surrounded by an oppositely polarized ring magnet 54. Unillustrated mechanisms scan the magnetron 52 circumferential and radially about the chamber axis 58 so as to increase the uniformity of sputter deposition and to reduce wastage of the target.
The magnetron 52 is designed to produce a very high magnetic field in the high-density plasma region 38 so as to produce a plasma region 38 of even higher density. The plasma density is so high that no argon working gas is required to sustain the plasma, but the copper ion density in the high-density plasma region 38 is sufficiently high for the copper ions to themselves resputter the target 14. It should be mentioned that conventionally a plasma can be induced and sustained in the parallel-plate configuration of either FIG. 1 or FIG. 2 only if the gas pressure is maintained above a minimum value in the milliTorr range. For sustained self-sputtering, once the plasma has been ignited, the controller 30 can turn off the argon supply valve 26, and the chamber pressure is reduced to an extremely low residual value, 10.sup.-8 Torr vs. 10.sup.-3 Torr and above in conventional sputtering.
Sustained self-sputtering offers a number of advantages. The possibility of argon being included in the sputter deposited material is eliminated. The high ionization fraction of the sputtered copper particles allows directional control of the copper being sputter deposited, thereby allowing better hole filling. For example, a grid anode 60 may be placed between the target 14 and the wafer 16 to act as the grounding plane for the sputter plasma. Since the grid anode 60 is in planar opposition to the target 14, the electrostatic attraction is nearly perpendicular to the wafer 16 rather than curved to the conventional grounding plane of the cylindrical shield 20. A DC power supply 62 controls the voltage of the grid 62 to a small positive value with respect to the pedestal 18. An additional variable power supply 66 biases the pedestal 18 independently of the grounded shield 20 and thus controls the velocity of the copper ions incident on the wafer 16. The power supply 66 for the pedestal 18 may be an RF power supply to control the plasma sheath voltage with a DC self-bias or may be a DC power supply or ground to provide direct DC control relative to the grid 60 or may be combination of RF and DC supplies.
As alluded to above and described in more detail by Fu et al., sustained self-sputtering requires an ignition sequence. The controller 30 opens the mass flow controller 26 between the argon source 24 and the vacuum chamber to build up sufficient argon pressure in the chamber to ignite a conventional plasma. Once the plasma has been ignited, the controller 30 turns off the mass flow controller 26 to stop the flow of argon into the chamber. However, as described above, the plasma discharge sustains itself after the interruption of argon because of the very high copper plasma density in the region region 38 adjacent to the high-field magnetron 52.
In commercial operation of this type of equipment, reliability is a major concern. Semiconductor fabrication equipment is costly, and downtime must be minimized. The processing is being performed upon partially processed wafers in which substantial amounts of processing time and costs have been invested. Abnormalities in the equipment need to be immediately detected and if possible immediately corrected.
The electrical power supplied to a reactor is not completely reliable. Commercially supplied power is subject to major and minor excursions, an example of which would be lightning striking outside power lines. Power conditioning equipment is usually used to suppress the excursions, but it does not provide complete protection. The large amounts of power being used in sputtering makes it difficult to buffer longer events.
Other types of temporary malfunctions can occur. The biased target may occasionally electrically arc to the grounded chamber or shield. Although the discharge may be relatively short, it presents a low impedance path and carries a large amount of current sufficient to debias the target and extinguish the plasma.
For most type of equipment, an arc or a power glitch may cause temporary malfunctions, but once the excursions have disappeared the equipment immediately restarts. Sustained self-sputtering however presents added difficulties. The time constants involved in these plasma processes are very short, and the operating margins for sustained self-sputtering are small. A relatively small or short power glitch may be enough to extinguish the sustained self-sputtering. Once it has been stopped, return of normal electrical power conditions will not be sufficient to reignite the plasma. Accordingly, it is desirable to have a procedure that detects interruptions to sustained self-sputtering and preferably automatically restarts the sustained self-sputtering.